This invention relates to a method of treating wastewater by aeration with at least 50% oxygen in an integral circular plant.
In areas where small flows of wastewater require treatment it is desirable to employ integral plants, i.e., plants in which all components are enclosed in a single outer wall. The cost of material and fabrication are lower for a relatively small integral wastewater treatment plant than for a plant comprising physically separate elements. Moreover, integral plants are compact and require a small land area for installation; such a plant also has a potential for much more simplified overall design as compared to a non-integrated facility.
Notwithstanding the requirement of being relatively small, the integral plant must maintain the desired level of wastewater treatment, i.e. the geometries of the constituent segments must promote good performance. For example, the mixing or aeration segments must promote efficient flow patterns and distribution of contained liquor; the clarifier must promote a low BOD content - effluent water and thickening of sludge underflow.
The prior art has made extensive use of circular plants for relatively small wastewater flows as they offer several advantages over other configurations such as rectangular. By providing a minimum perimeter to cross sectional area ratio, circular design tends to minimize material requirements for fabrication of the integral plant while promoting a highly efficient component arrangement. Additionally, construction costs may be less in some instances for circular geometries than for other shapes, as for example in concrete fabrication.
The prior art has employed biological treatment processes in small circular plants, primarily because of their applicability to a wide variety of wastewaters and effluent requirements and comparatively low capital cost. The major biological treatment process in commercial use is based on activated sludge, in which wastewater is mixed in an aeration zone with oxygen-containing gas and the activated sludge. The latter consists essentially of aerobic organisms which in the presence of dissolved oxygen, absorb and assimulate the biochemically oxidizable organic content (BOD) of the wastewater, converting the organic material to forms which can readily be separated from the purified water in the clarification zone. Under normal conditions the organisms multiply rapidly in the aeration zone and when the requisite period of BOD conversion is complete, the mixed liquor is settled in a clarifier zone and the purified effluent decanted to receiving waters. Sludge is withdrawn from the bottom of the clarifier zone with part thereof being recycled to the aeration zone to maintain effective biological action on the influent wastewater.
Until very recently atmospheric air has been the sole source of oxygen in activated sludge plants. But in recent years this system has been vastly improved by the use of high purity oxygen gas as the oxidant in a series of closed rectangular tanks, preferably with staging of gas and liquor from tank to tank in the manner described in the U.S. Pat. Nos. 3,547,813, 3,547,814 and 3,547,815 all to J. R. McWhirter. The high purity oxygen aerated systems offer important advantages over air aerated plants as for example higher levels of biological action on influent wastewater therefore smaller aeration tanks.
The operation of clarification is greatly influenced by the type of aeration employed. Clarifiers in the activated sludge process have two functions: They must provide an effluent with a low level of suspended solids and must also thicken sedimentary solids and provide a sludge of sufficient concentration to maintain effective biological action in the aeration zone. The efficiency of the clarifier in performing these two functions depends largely on the physical nature of the solids in the liquor discharged from the aeration zone and here again the oxygen aeration process has distinct advantages over air aeration systems. The latter produces typically small-sized fragile, relatively unflocculated solids particles which do not settle well in the clarifier. Moreover, the settled sludge possesses a high specific volume as for example measured by the Sludge Volume Index (SVI) so that because of the poor settling characteristics and compactibility, a clarifier processing air aerated sludge must be comparatively large in size to insure adequate performance. Oxygen aeration systems by contrast produce sludge with superior settling characteristics, i.e., higher settling velocities, (lower SVI) and better dewatering ability.
In sizing clarifiers the dual functions of clarification and thickening must be separately considered and an overall area chosen which accomodates both requirements. It is further necessary to develop a clarifier design which is free from stagnant areas or short-circuiting flows. This is accomplished by providing a geometric form without sharp corners or regions inaccessible to the liquor flow, and uniform fluid flow patterns within the clarifier. Although the latter characteristic is primarily insured by distributing the influent liquor as uniformly as possible over the entire cross sectional area of the clarifier, it is also necessary to provide liquor flow patterns within the vessel which permit sufficient liquor residence time for sedimentation to occur. It is also desirable to provide a liquor flow path in the clarifier which brings the influent strength to a relatively quiescent state and thus minimize fluid velocities within the bulk fluid volume.
To effectively use the entire area provided in the clarifier, the length of the liquor flow path must be at least equal to and at peak flow conditions preferably identical with the path length necessary for sedimentation. If the sedimentation path is shorter than the actual path provided for liquor travel then distribution of solids will occur over only part of the clarifier area. Under these circumstances the clarifier has been over designed and the integral plant is larger than necessary. If the sedimentation path is longer than the actual path provided for liquor travel then a gross loss of solids may occur in the clarifier effluent. Unfortunately, the prior art air aeration integral circular plants with arcuate clarifier zones require the zone to extend around the entire periphery of the outer wall, i.e., 360.degree., for the liquor flow path length to be at least equal to the sedimentation path length. That is, foreshorting of the clarifier arc to less than the full circumference causes the sedimentation path length to exceed the actual liquor flow path length and substantially reduce the solids-liquid separation in the clarifier. This means that only the central portion of an air aerated integral circular plant is available as the aeration zone and the plant must be sized on the basis of the required aeration zone volume. The result of this severe limitation is a relatively large plant to process a given wastewater flow rate.
An object of this invention is to provide an improved method of and apparatus for biological treatment of wastewater in an integral circular plant.
Another object of this invention is to provide an activated sludge type system employing high purity oxygen aeration for relatively low wastewater flow rates in an integral circular plant which is substantially more compact than rectangular configuration plants.